The present invention relates to methods and apparatuses for heating electric motors and adjacent fluids.
Electric motors, such as electric compressor motors for refrigeration units, often operate over a range of ambient temperature conditions. During relatively low ambient temperature operation, compressors often cycle on and off due to limited load demand. During the compressor off time, temperatures of fluids associated with the refrigeration unit and compressor, such as oil and refrigerant, can be very low. Such low fluid temperatures can, for instance, affect oil delivery at compressor start up and reduce compressor reliability. In addition, if the refrigeration unit shuts down for an extended period of time, typically longer than about six hours, the liquid refrigerant starts to migrate to the compressor, which is generally the most massive component in the system. The presence of refrigerant in the compressor at start-up produces what is known as a “flooded start”. When the compressor starts in a flooded condition, the liquid refrigerant in the compressor causes high stress for the compressor and other components in the system and therefore reduces reliability.
It is thus desirable to heat refrigerant and oil under low ambient temperature conditions to facilitate reliable operation of a compressor. One existing solution is to use an external electrical crankcase heater to heat the refrigerant and oil by heat transfer through the compressor base and shell (see, e.g., U.S. Pat. Nos. 3,133,429; 4,066,869; 4,755,657; and 5,062,277). However, this known solution presents a number of problems. An external element increases the number of components in the refrigeration unit. These external heaters also require proper installation using a heat sinking compound. During use, the external heater must resist moisture and corrosion during thermal cycling, which can make construction and maintenance problematic. Also, these external heaters can result in inefficient transfer of heat to refrigerant and oil in a compressor.